1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and in particular, it relates to a method of manufacturing a thin, flexible (having a flexible properly) semiconductor device. Further, the present invention relates to a method of reducing parasitic capacitance which is generated between wirings formed on differing layers through insulating films. Note that the term semiconductor device in this specification indicates general devices which function by utilizing semiconductor properties, and that in particular, the present invention can be suitably applied to integrated circuits using elements having SOI (silicon on insulator) structures in which a semiconductor layer is formed on an insulator, to active matrix liquid crystal display devices structured using thin film transistors (TFTs), to active matrix EL display devices, and the like. The term thin film device indicates an electronic device containing a thin film transistor (TFT) structured using a semiconductor thin film, and at least one element from amount elements such as wirings, conductive layers, resistors, and capacitive elements in this specification.
2. Description of the Related Art
Integrated circuits using elements having SOI structures in which semiconductor layers are formed on an insulator exist as one kind of semiconductor device. It is possible to have little parasitic capacitance, and to have high operation speed, by forming semiconductor layers on insulators.
One type of semiconductor device is an active matrix liquid crystal display device. Structures in which a substrate on which thin film transistors (TFTs) are formed and used as switching elements of pixels (TFT formation substrate), and a substrate on which an opposing electrode is formed (opposing substrate) are joined, and liquid crystals are injected in a gap between the substrates, is prevalent for active matrix liquid crystal display devices. The voltage applied to the liquid crystals can be controlled for each single pixel by the TFTs formed on a transparent substrate such as glass, and therefore active matrix liquid crystal display devices have clear images and are widely used in office automation equipment, televisions, and the like.
Further, active matrix EL display devices are known as one type of semiconductor device. Active matrix EL display devices have a structure in which an EL material is sandwiched between two electrodes, and an electric current flows, thereby causing light to be emitted. The electric current flowing in the EL material can be controlled for each single pixel by using a plurality of pixel transistors, and therefore an image is clear.
The level of integration for these types of semiconductor devices is increased and becoming minute. Parasitic capacitance which is generated between wirings of a semiconductor device leads to electric signal propagation delays, and this hinders high speed operation and accurate electric signal propagation. There are two types of parasitic capacitance, one which is generated between wirings formed on the same layer, and one which is generated between wirings formed on different layers through an insulation film.
If the level of integration is increased, the distance between wirings formed on the same layer becomes smaller, thereby increasing the parasitic capacitance. Wirings may be moved to different layers in order to reduce the parasitic capacitance between wirings formed on the same layer. Namely, the integration level of wirings on the same layer is spread among several layers. Lowering the parasitic capacitance which is generated between wirings formed on different layers, through an insulation film, contributes to an improvement in the overall integration level of the semiconductor device.
There are methods such as making insulation films thicker and increasing the distance between wirings, and using insulation films having a low dielectric constant, in order to reduce the parasitic capacitance generating between wirings formed on different layers through an insulation film. However, if the insulating film is made thicker, then not only does it become more difficult to form an opening portion in the insulation film in order to make a conductive connection between wirings, but there are also cases in which problems such as conductive layers formed by sputtering, for example, breaking in the inside of the opening portion, or being unable to ensure a sufficient film thickness, with the resistance therefore becoming large. Further, insulation films having low dielectric constants have a likelihood to develop problems relating to film quality, such as resistance to heat and permeability, and manufacturing problems such as dimensional changes due to etching. For example, although dependent upon the etching conditions, the hole diameter may become larger to approximately 1 μm for a case in which a 1 μm thick acrylic is used, and there may be damage in improving the overall level of integration of the semiconductor device.
In addition, there is a method in which the formation order of the conductive layers used for forming the wirings is changed. For a case structuring integrated circuits, having two layers of wirings for making the conductive connection between elements, by top gate transistors, the following order of formation is normally used: active layer; first insulation film (gate insulation film); first conductive layer (gate electrode); second insulation film (first interlayer insulating film); second conductive layer (first wiring); third insulation film (second interlayer insulating film); and third conductive layer (second wiring).
If the structure is changed to the following; first conductive layer (second wiring): first insulation film (lower portion insulation film); active layer; second insulation film (gate insulation film); second conductive layer (gate electrode); third insulation film (first interlayer insulating film); and third conductive layer (first wiring); then the distance between the first wiring and the second wiring becomes large, and the parasitic capacitance generating between the wirings can be reduced.
The distance between the first wiring and the second wiring becomes large in this case, and problems relating to openings and the conductive connection can be prevented through the active layer, for example. However, even with the same second wiring, with the latter case a material able to withstand the film formation temperature of the subsequently formed active layer and the thermal activation temperature of injected impurities must be used, and the same materials cannot always be used by the former and latter cases. For example, Al is often used as a wiring material having a low resistivity, but its resistance to heat is low, and it cannot be used in the latter case.
Note that, within this specification, an electrode is a portion of a wiring, and the terms wiring and electrode are used separately for convenience. However, the term wiring is always contained within the word electrode.
Semiconductor devices like those stated above are recently being used in portable devices and the like, and there are demands to make the portable devices thinner, lighter, and more flexible (flexible property). The major portion of the thickness of a semiconductor device is the thickness of its substrate, and the substrate may be made thinner in order to make the portable device thinner and lighter. However, if the substrate is made thinner, then manufacturing becomes difficult due to trouble in photolithography processes caused by warping of the substrate during manufacture, and substrate breakage more easily occurring during transportation of the substrate. A light, flexible display device can be manufactured, provided that a semiconductor device can be manufactured on a transparent plastic substrate or the like, but this has not yet been accomplished due to problems such as the heat resistance of plastic substrates.
Further, high speed operation of electric circuits and accurate propagation of electric signals can be performed for reducing the parasitic capacitance which is generated between wirings formed on different layers through an insulation film, thereby being able to use wiring materials having a low thermal resistance, such as Al, which have not been able to be used.